MAINTAINERS: add entry for idmapped mounts

I'd like to continue maintaining the work that was done around idmapped,
make sure that I'm Cced on new patches and work that impacts the
infrastructure.

Link: https://lore.kernel.org/r/20220203131411.3093040-3-brauner@kernel.org
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>